To evaluate whether the knockdown of RAD21 sensitized ovarian cancer cells to PARP inhibitors by impairing DNA damage repair, we treated siRNA- or lentivirus-transfected cells with PARP inhibitors for 4 days at IC50 concentrations and quantified DSB formation based on the presence of γ-H2AX foci. The gene discussed is RAD21; the disease is ovarian cancer.